Documentation

A library to deal with application settings.
This library deals with read-write application settings.
You will have to specify the settings that your application
uses, their name, types and default values.
Setting types must implement the Read and Show typeclasses.

The settings are saved in a file in an INI-like key-value format
(without sections).

Reading and updating settings is done in pure code, the IO
monad is only used to load settings and save them to disk.
It is advised for the user to create a module in your project
holding settings handling.

If you do it, saveSettings will also save settings
which have not been modified, which are still at their
default value in the configuration file, in a commented
form, as a documentation to the user who may open the
configuration file.
So for instance if you declare this default configuration
and have set the font size to 16 but left the other
settings untouched, the configuration file which will be
saved will be:

fontSize=16
# dateFormat="%x"
# backcolor=(255,0,0)

If you did not specify the list of settings, only the
first line would be present in the configuration file.

With an ordinary setting, one row in the configuration file
means one setting. That setting may of course be a list
for instance. This setup works very well for shorter lists
like [1,2,3], however if you have a list of more complex
items, you will get very long lines and a configuration
file very difficult to edit by hand.
For these special cases there is also the ListSetting

Which is much more handy for big lists. An empty list is represented
like so:

testList=

There is also another technique that you can use if you have too long
lines: you can put line breaks in the setting values if you start the
following lines with a leading space, like so:

testList=["list1",
"list2", "list3"]

In that case don't use the ListSetting option. Any character after the
the leading space in the next lines will go in the setting value. Note
that the library will automatically wrap setting values longer than 80
characters when saving.

Once we declared the settings, we can read the configuration
from disk (and your settings module should export your wrapper
around the function offered by this library):

Information about the default configuration. Contains
all the settings (that you declare using getDefaultConfig)
and their default values. It is useful when you save a
configuration file, if you give this information to saveSettings,
it will save default options in the configuration file
in a commented form, as a form of documentation to a user
who would edit the configuration file.
However this is completely optional, you can give
emptyDefaultConfig if you don't want this behaviour.

The type of a setting.
It contains the setting name
(key in the configuration file) and its default value.

It is advised to have a module in your project handling settings.
In this module, you'd have all the settings declared at the
toplevel, and exported.
The rest of the application can then do

getSetting <setting>
setSetting <conf> <setting> <value>

and so on.

Setting declares a simple setting. A value for that setting will be stored
in the configuration file in a single line.

ListSetting however declares a list setting.
While it is perfectly fine to store lists
using the usual Setting constructor, if you have a list
of more complex items, you will get very long lines and a
configuration file very difficult to edit or review by hand.

The ListSetting will store settings using one line per item
in the list:

Read settings from disk.
Because it is doing file I/O it is smart to wrap the call
with a try, as I/O exceptions can be thrown.
Also, the function will throw a ParseException if the
file is not properly formatted.
NOTE that if the file is properly formatted in general,
but a value is stored in an invalid format (for instance "hello"
for a Double), you will get no error and get the default value
for that setting when you attempt to read it.

This function returns a pair. The first element
is the configuration itself, which you can use to save
back or modify the configuration.
The second element is a function wrapped in the GetSetting
newtype. This function allows you to read a configuration
option simply by giving that option (without that callback
you'd have to call getSetting settings <setting>, so
the callback lets you save a parameter).
There is no such shortcut for setSetting though, as it's
normally used less often and in other contexts, it is probably
OK to have that extra parameter for the setSetting.

Most of the time you can use the second function you get from readSettings,
wrapped in a GetSetting newtype, however sometimes it's nicer to just pass
a single Conf to other functions if you're going to read or
write to the configuration. The GetSetting lets you only read.